Gasolines are generally composed of a mixture of hydrocarbons, boiling at atmospheric pressure in a very narrow temperature range, e.g., 77° F. (25° C.) to 437° F. (225° C.). Gasolines are typically composed of mixtures of aromatics, olefins, and paraffins, although some gasolines may also contain such added non-hydrocarbons as alcohol (e.g., ethanol) or other oxygenates (e.g., methyl t-butyl ether (“MTBE”). Gasolines may also contain various additives, such as detergents, anti-icing agents, demulsifiers, corrosion inhibitors, dyes, deposit modifiers, and octane enhancers. The presence of oxygen in the fuel tends to raise the effective air-to-fuel ratio for combustion and fuel oxygen may effect catalyst efficiency. While the oxygen in ethanol can raise this air-to-fuel ratio which may increase combustion temperature, the lower temperature of combustion for ethanol mitigates this effect. The oxygen in ethanol also reduces carbon monoxide (“CO”) and volatile organic compound (“VOC”) emissions during high-emissions conditions in new vehicles and during all conditions for vehicles that do not have operational oxygen sensors or catalysts.
The passage of the Clean Air Act (“CAA”) Amendments of 1990 has impacted all major transportation fuels in the United States and stimulated research into using alternative motor fuels that include oxygenates. In order to comply with the CAA, gasoline marketers admixed oxygenates into gasoline, but also changed the hydrocarbon composition by altering the content of benzene, total aromatics, butane, total olefins, and similar components. These considerations affect the reactivity of new gasolines and translate into the performance characteristics of admixed oxygenates, i.e., distillation, volatility, azeotropic behavior, oxidation stability, solubility, octane values, vapor pressure, and other gasoline characteristics known to those skilled in the art.
Research regarding oxygenated fuel substitutes and components has focused on aliphatic alcohols and ethers, including, but not limited to, methanol, ethanol, isopropanol, t-butanol, MTBE, ethyl t-butyl ether (“ETBE”), and t-amyl methyl ether (“TAME”). Most research has focused on using MTBE in gasoline formulation. Generally, oxygenate gasoline components have been blended into gasoline separately. However, there have been mixtures of such components disclosed, such as blends of gasoline containing components other than ethers, such as alcohols.
Historically, gasoline has usually embodied pressures of between about 9 to about 15 pounds per square inch (“PSI”) of pressure. Recent evaporative emission regulations have forced the reduction of gasoline vapor pressures. Ether components provide advantageous vapor pressure blending characteristics for these gasolines. In the late 1990s, the CAA has now caused refiners to reformulate gasoline to achieve vapor pressures of about 7.5 to about 8.5 PSI. This is because the CAA is trying to reduce vehicle emissions that constitute air toxins and participate in the formulation of air pollution (“smog”), for example, CO, NOx, and VOCs. These lower vapor pressure requirements motivated the use of MTBE. It has been used in “premium” gasoline since 1979 as a high-octane additive to function as an oxygenate. In fact, MTBE has replaced lead and other highly contaminating additives such as benzene, toluene, ethylbenzene, and xylenes (“BTEX”).
MTBE is an ether—having relatively low odor and taste thresholds compared to other organic compounds. MTBE's odor threshold in water is between about 45 and about 95 parts per billion (“ppb”). Its taste threshold in water is about 134 ppb. As a result, MTBE can be detected in drinking water through odor and taste at relatively low concentrations. Ultimately, MTBE is encountered through drinking contaminated water, use of the water in cooking, and inhalation during bathing.
Vast amounts of MTBE-containing gasoline are stored in underground storage tanks (“UST”), which have been known to leak. Seepage of MTBE from leaky tanks into groundwater and spillage of MTBE during tank filling operations and transfer operations at distribution terminals have led to considerable contamination of groundwater near these tanks. Because MTBE is highly soluble in water—about 43,000 parts per million (“PPM”)—MTBE may be found as plumes in groundwater near service stations, related storage facilities, and filling terminals throughout the United States. See Steffan. Therefore, a need exists to exploit alternative sources of oxygenates as gasoline additives.
To this end, ethanol has been used as an alternative to MTBE in gasoline-oxygenate blends wherein the vapor pressure and emission requirements were less restrictive. Ethanol has some properties that are different than MTBE. However, ethanol blends have nearly twice the fuel-oxygen content of the MTBE blends. Furthermore, these ethanol blends exhibit as much as about 1 PSI higher Reid Vapor Pressure (“RVP”) volatility absent pre-adjustment of the base clear-gasolines to accommodate this volatility. Accordingly, there exists a need to use an alternative to MTBE that provides acceptable volatility.
With mounting pressures being attributed to the use of ethers, ethanol continues to find increasing application in low-RVP gasolines. Cost-effective blending of “Specification,” ethanol-containing gasoline require special characteristics and preparations. Substantiating this need, the 23-state Governors' Ethanol Coalition released a report entitled “The Fate and Transport of Ethanol-Blended Gasoline in the Environment” in 1999 that confirms that ethanol, a renewable oxygenate added to gasoline to make it burn cleaner, poses no threat to surface water and ground water. The report confirmed that, in California, more than 10,000 wells have been contaminated by MTBE and its pungent odor renders water undrinkable. In 1999, California Governor Gray Davis called for eliminating the use of MTBE in the state's gasoline. In fact, Executive Order D-5-99 requires a phase out and eventual elimination of MTBE use by Dec. 31, 2002. Accordingly, a need exists to reduce or replace MTBE additives in gasoline to make a cleaner burning fuel that still provides acceptable temperature and volatility characteristics.